The process of the present invention is particularly applicable, but not necessarily restricted to, the manufacture of low-alloy ferrous parts such as gears, clutch components, races of tapered roller bearings or the like which are characterized as having working portions or surfaces that are subjected to concentrated loads, torques, or other high stresses. In order to provide satisfactory performance and durability of such parts, it has heretofore been conventional to fabricate the entire part from a suitable high performance alloy including those portions which are not subjected to high loading and stresses. In order to conserve on the use of such relatively high cost high performance alloys, it has also been heretofore proposed to fabricate composite articles incorporating the high performance alloy in those portions subjected to high loading and stresses while the remaining portion of the part is comprised of less expensive alloys. A powder metallurgical process of the last mentioned type is disclosed in U.S. Pat. No. 3,727,999 which is assigned to the same assignee as the present invention. While the fabrication of such composite parts provide for satisfactory performance and durability, the steps and tooling required for forming a composite component is somewhat costly, time consuming and labor intensive.
In order to overcome the foregoing problems, it has also been proposed to fabricate dense metal parts employing powder metallurgical techniques in which the sintered blank is subjected to carburizing over the entire surface thereof whereafter the carburized part is forged to final dimensions. A process of the foregoing type is disclosed in U.S. Pat. No. 3,992,763 which also is assigned to the present assignee. While the process described in the aforementioned U.S. Patent, to which reference is made for further details, has provided satisfactory parts, a continuing difficulty has been encountered in the final forgoing and machining operations of the part due to the presence of a carburized case over all portions of the parts. The presence of such a hard case necessitates the use of more expensive tooling during the final machining and finishing operations detracting somewhat from the economy in the manufacture of such parts.
The process of the present invention overcomes many of the problems and disadvantages associated with the aforementioned and other prior art powder metallurgical techniques in providing a process in which only selected portions of a part are carburized leaving remaining portions substantially uncarburized providing therewith a final part possessed of the requisite mechanical properties and performance which can be produced at commercial production rates and can be finish machined employing conventional equipment.
Furthermore, irrespective of any machining considerations, the process of the present invention provides an uncarburized surface on an otherwise carburized part which thus allows the possibility of electron beam welding or similar application which requires a low carbon surface.